Motor vehicles having various control systems require a determination of vehicle speed, that is then provided to these systems. One system, which requires the vehicle speed as an input signal is, for example, is a longitudinal control system that controls the speed of the vehicle by setting an acceleration value to preset desired or theoretical speed. Additionally, longitudinal control systems enhanced by distance control, which are known as distance-related longitudinal control systems, are also obtainable today from some manufacturers. Such systems, offered, for example by BMW, the Assignee of the present invention under the name of “Active Cruise Control”, make it possible to automatically drive the motor vehicle while maintaining a desired distance from the vehicle ahead, at a desired speed or at an appropriately slower speed. In principle, the generally known longitudinal control or vehicle speed control systems, which maintain a specific predetermined speed, is enhanced by an additional distance function, such that the use of an “active” driving speed control of this type is also possible in heavy motorway and non-motorway road traffic. The system is therefore able to adjust the vehicle's speed to suit the surrounding traffic conditions.
The currently available longitudinal control systems or distance-related longitudinal control systems only control the speed down to a lower limiting speed of, for example 30 km/h. At such speeds, it is sufficient to determine the vehicle speed from the average wheel speed of the non-powered wheels. For average wheel speeds of the non-powered wheels that exceed a predetermined positive average threshold of, for example 0.4 km/h, the determined vehicle speed fairly accurately corresponds to the average wheel speed of the non-powered wheels. The resolution of this calculation is usually within the range of 0.1 km/h. Speeds for which the average of the wheel speeds is less than the predetermined average threshold are always output as zero, and negative speed values are not output. For longitudinal control systems, which are only operational down to a minimum speed of approximately 30 km/h, this is quite adequate, since on the one hand very low speeds are not required and on the other hand the resolution is also sufficiently low.
More recently, however, distance-related longitudinal control systems have been developed and used in vehicles and have been enhanced by a stop-and-go function, thus allowing a distance-related longitudinal control until the vehicle comes to a standstill and starts up again. In these longitudinal control systems, a reliable detection of a standstill state of the motor vehicle is necessary. If detecting the standstill state was only determined using the vehicle speed method mentioned above, where the average of wheel speed of the non-powered wheels is just under the average threshold, a standstill would be detected although the vehicle is still moving, ever so slightly. Additionally, is the vehicle is rolling back; a standstill state would also be detected. This imprecise and possibly premature standstill detection would result in the motor vehicle being stopped with a clearly noticeable jolt.
DE 195 03 270 A1 discloses a method for determining the standstill time of a vehicle, in which during a braking phase for at least one wheel, a threshold speed value is established, then at several successive times, in order to achieve a measurable minimum speed, at least one further speed value is detected for the wheel or wheels and the time of the vehicle standstill is estimated by forming one or more speed gradients between the detected speed values and by extrapolating them to zero speed. A method of this type is laborious and is based merely on an estimate.
Furthermore, so-called hill holder functions are well known, which are intended to hold the motor vehicle in a position stationary when on a hill. In order to detect a standstill position on the hill, the rotational direction of at least the non-powered wheels is evaluated, in addition to the speed of the vehicle. Thus, WO 93/04888 A1 discloses a standstill control device in which to detect a standstill position of a motor vehicle, the signals of a rotational direction sensor and the rotational speed of at least one wheel are observed and evaluated. An evaluation of this type is only expedient in the case of very slow vehicle speeds.